A fuel cell is basically composed of a polymer electrolyte membrane selectively transporting hydrogen ions and a pair of catalyst electrodes (a fuel electrode and an air electrode) sandwiching the polymer electrolyte membrane therebetween. The fuel cell having the foregoing configuration can produce continuously electrical energy from a fuel gas (containing hydrogen) supplied to the fuel electrode (anode) and an oxidizing gas (containing oxygen) supplied to the air electrode (cathode).
The catalyst electrode is disposed on the side of the polymer electrolyte membrane. The catalyst electrode is composed of a catalyst layer promoting a redox reaction of the catalyst electrode and a gas diffusion layer that is breathable and electrically conductive, disposed on the outer side of the catalyst layer. Further, the gas diffusion layer is composed of a carbon coat layer disposed on the side of the catalyst layer, which facilitates the contact to the catalyst layer, and a gas diffusion base material layer which diffuses a gas supplied from the outside and supplies the gas to the catalyst layer. The unified body of the polymer electrolyte membrane and the pair of the catalyst electrodes (including catalyst layer, carbon coat layer, and gas diffusion base material layer) is called a membrane electrode assembly (hereinafter called as “MEA”).
MEA can be electrically connected in series by stacking. When MEA is stacked, an electrical conductive separator is disposed between each MEA so as not to mix the fuel gas and the oxidizing gas, and to electrically connect each MEA. The assembly given by sandwiching MEA between a pair of the separators is called “fuel cell” or simply “cell”. A stack product of plural fuel cells is called “fuel cell stack” or simply “stack”.
In the fuel cell, a gas flow channel is formed in the face of the separator in contact with the catalyst electrode so as to supply a reactive gas (fuel gas or oxidizing gas) to the catalyst electrode and to discharge excess gas and excess water. The gas flow channel formed on the separator is generally composed of plural straight parallel-flow channels that are communicated with tubes called as a manifold running through the fuel cell stack. The manifold supplies the reactive gas to the gas flow channel and discharge excess gas and water out of the gas flow channel for all of the fuel cells in the fuel cell stack.
In general, the cell or stack is sandwiched by collector plates, insulator plates, and terminal plates and provides a fuel cell configured in a form used generally.
In the fuel cell having the foregoing configuration, when the fuel gas containing hydrogen is supplied to the fuel electrode and the oxidizing gas containing oxygen is supplied to the air electrode, electrical energy can be produced in accordance with the reactions mentioned below.
The hydrogen supplied to the fuel electrode diffuses through the gas diffusion layer of the fuel electrode and reaches the catalyst layer. In the catalyst layer, the hydrogen is separated into a hydrogen ion and an electron. The hydrogen ion is transported to the air electrode through the polymer electrolyte membrane in a water-retaining state. The electron is transported to the air electrode through an external circuit. The electron passing through the external circuit can be used as electrical energy. In the catalyst layer of the air electrode, the hydrogen ion transported through the polymer electrolyte membrane reacts with the electron transported through the external circuit and the oxygen supplied to the air electrode, whereby water is generated.
The fuel cell, as mentioned above, generates water by the power generation reaction. The power generation efficiency of the fuel cell decrease when the inside of the cells is excessively wetted with water, so that the water generated on the power generation is discharged to the outside with the help of the gas that flows the flow channel formed on the separator.
A material of perfluorosulfonic acid is used in many cases for the polymer electrolyte membrane through which the hydrogen ion is transported. The polymer electrolyte membrane is ion conductive when the membrane retains water sufficiently, but it loses the ion conductivity under drying condition. Therefore, in order to allow the power generation reaction to proceed efficiently over the entire face of the fuel cell, it is required to prevent the inside of the cell from being dried and to keep uniform in-plane water distribution in the cell.
In order to prevent the inside of the cell from being dried and to keep uniform the in-plane water distribution in the cell, an external-humidifying method has been employed so far, where the inside of the cell is humidified from the outside. By the external-humidifying method, an external-humidifier supplies a reactive gas which has a dew point higher than the internal temperature of the fuel cell, and makes the inside of the cell being over-humidified. However, this method has a disadvantage of causing easily flooding in which the supply of the reactive gas to the catalyst layer is prevented, because water droplets are generated in the gas diffusion layer. In addition, the method has another disadvantage of having difficulty in reducing the cost of the fuel cell system, because the external humidifier is required. Furthermore, the method has another disadvantage of having difficulty in driving the fuel cell at a high temperature where a high efficiency of power generation is expected, because the internal temperature of the fuel cell is required to be lower than the reactive gas dew point attainable by the external humidifier.
As a method that addresses the foregoing disadvantages of the external humidifying method, there has been proposed an internal humidifying method through which the inside of the cell is humidified by diffusing the water generated by the power generation reaction into the cell. However, in the internal humidifying method, the polymer electrolyte membrane placed near the inlet of the reactive gas tends to dry, because the reactive gas supplied from the outside is dry. On the other hand, the polymer electrolyte membrane placed near the outlet of the reactive gas becomes excessively wetted with water in many cases, because the reactive gas that passes through the gas flow channel contains the water generated by the power generation reaction. In the internal humidifying method, the in-plane water distribution in the cell becomes non-uniform and the power generation reaction proceeds mainly on the outlet side of the reactive gas. In this way, the internal humidifying method still has a disadvantage of lowering the power generation efficiency as a whole cell.
As a method that addresses the foregoing disadvantage of the internal humidifying method, there has been proposed a method in which a reactive gas supply manifold is placed adjacent to a reactive gas discharge manifold (for example, refer to Patent Document 1).
FIG. 1 shows a front view (perspective view) of a fuel cell in accordance with Patent Document 1. The structure of a separator on a fuel electrode side is drawn with solid lines and the structure of a separator on an air electrode side is drawn with dashed lines. In FIG. 1, a fuel gas supply manifold 10 and a fuel gas discharge manifold 12 are disposed in such a manner that they are adjacent to each other, and they are communicated with each other through a fuel gas flow channel 14 having a rectangular geometry. Similarly, an oxidizing gas supply manifold 20 and an oxidizing gas discharge manifold 22 are disposed in such a manner that they are adjacent to each other, and they are communicated with each other through an oxidizing gas flow channel 24 having a rectangular geometry. The above configuration allows the water near the outlet of the reactive gas (on the side of the reactive gas discharge manifold) to be transported to the inlet side of the reactive gas (on the side of the reactive gas supply manifold) through an electrolyte membrane. In this way, the gas on the outlet side can be prevented from being excessively wetted, and the gas on the inlet side can be prevented from being dried.
As mentioned above, by disposing the reactive gas supply manifold and the reactive gas discharge manifold in such a manner that they are adjacent to each other, the water can be transported through the electrolyte membrane in the in-plane direction thereof, whereby the in-plane water distribution of the polymer electrolyte membrane can be made uniform.
However, the fuel cell of Patent Document 1 has a disadvantage of requiring complex structures in the cell and the stack in order to make the cell area large. That is, the fuel cell of Patent Document 1 is required to increase the number of manifold and the accompanied structures thereof when the side length of the cell becomes larger, because the reactive gas supply manifold and the reactive gas discharge manifold are disposed in such a manner that they are adjacent to each other in each of the rectangular gas flow channel (see FIG. 1). Therefore, when the cell area is made large, the structures of the inside cell and stack become complex, whereby the production cost of the fuel cell becomes high.
Furthermore, the fuel cell of Patent Document 1 also has a disadvantage of having a low capability of transporting the water in an in-plane direction. That is, the water can be transported more efficiently between the outlet side and the inlet side of the reactive gas when the difference in the amount of the water retained in each side is larger. In order to increase the amount of the water on the outlet side of the reactive gas, the length of the reactive gas flow channel may be made larger. However, in the fuel cell of Patent Document 1, because the gas flow channel has a rectangular geometry, the length of the gas flow channel is limited by the side length of the cell and the distance between the manifolds (refer to FIG. 1). Therefore, the fuel cell of Patent Document 1 has a low capability of transporting the water in an in-plane direction.
As a method that addresses the above disadvantages, for example, there is the one that is disclosed in Patent Document 2.
FIG. 2 shows a front view of a separator on the side of an air electrode of a fuel cell in accordance with Patent Document 2. In FIG. 2, an oxidizing gas supply manifold 1a and an oxidizing gas discharge manifold 1b are disposed in such a manner that they are adjacent to each other. And more, a flow channels 25 are meander-shape forward and return flow channels, and the forward flow channel and the return flow channel are adjacent to each other. This configuration allows reduction of the number of manifolds. As a result, the structure of the cell and stack hardly becomes complex when the area of the fuel cell is increased. The ribs that determine the outline of the flow channel are porous and they have capillaries connecting the forward flow channel with the return flow channel. This configuration allows the water on the outlet side of the reactive gas (on the side of the reactive gas discharge manifold) to be transported to the inlet side of the reactive gas (on the side of the reactive gas supply manifold). In this way, the in-plane water distribution of a polymer electrolyte membrane can be made uniform.
In order to make uniform the partial pressure of the reactive gas on the inlet side of the reactive gas (on the side of the reactive gas supply manifold) and the partial pressure of the reactive gas on the outlet side of the reactive gas (on the side of the reactive gas discharge manifold), there has been proposed a method in which the cross-sectional area of a reactive gas flow channel is decreased from upstream side to downstream side at a set ratio (for example, refer to Patent Document 3). In Patent Document 3, the reactive gas flow channel is composed of plural channels which are connected in parallel. The number of parallel flow channels of the downstream part is smaller than the number of parallel flow channels of the upstream part in such a manner that the cross-sectional area in the downstream part is decreased as compared with the cross-sectional area in the upstream part. Furthermore, in Patent Document 3, the gas flow channel is snaked in order to decrease the number of the manifold.    Patent Document 1: Japanese Patent Laid-Open Publication No. 2002-151105    Patent Document 2: Japanese Patent Laid-Open Publication No. 2003-109620    Patent Document 3: Japanese Patent Laid-Open Publication No. S56-134473.